nixos/btrfs: add services.btrfs.autoScrub

for automatic regular scrubbing of mounted btrfs filesystems,
similar to what's already there for zfs.

1 files changed, 105 insertions, 8 deletions

diff --git a/nixos/modules/tasks/filesystems/btrfs.nix b/nixos/modules/tasks/filesystems/btrfs.nix
index 8cfa1b6921d3..1384873b6631 100644
--- a/nixos/modules/tasks/filesystems/btrfs.nix
+++ b/nixos/modules/tasks/filesystems/btrfs.nix
@@ -1,35 +1,132 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, utils, ... }:
 
 with lib;
 
 let
 
   inInitrd = any (fs: fs == "btrfs") config.boot.initrd.supportedFilesystems;
+  inSystem = any (fs: fs == "btrfs") config.boot.supportedFilesystems;
+
+  cfgScrub = config.services.btrfs.autoScrub;
+
+  enableAutoScrub = cfgScrub.enable;
+  enableBtrfs = inInitrd || inSystem || enableAutoScrub;
 
 in
 
 {
-  config = mkIf (any (fs: fs == "btrfs") config.boot.supportedFilesystems) {
+  options = {
+    # One could also do regular btrfs balances, but that shouldn't be necessary
+    # during normal usage and as long as the filesystems aren't filled near capacity
+    services.btrfs.autoScrub = {
+      enable = mkEnableOption "Enable regular btrfs scrub";
 
-    system.fsPackages = [ pkgs.btrfs-progs ];
+      fileSystems = mkOption {
+        type = types.listOf types.path;
+        example = [ "/" ];
+        description = ''
+          List of paths to btrfs filesystems to regularily call <command>btrfs scrub</command> on.
+          Defaults to all mount points with btrfs filesystems.
+          If you mount a filesystem multiple times or additionally mount subvolumes,
+          you need to manually specify this list to avoid scrubbing multiple times.
+        '';
+      };
 
-    boot.initrd.kernelModules = mkIf inInitrd [ "btrfs" "crc32c" ];
+      interval = mkOption {
+        default = "monthly";
+        type = types.str;
+        example = "weekly";
+        description = ''
+          Systemd calendar expression for when to scrub btrfs filesystems.
+          The recommended period is a month but could be less
+          (<citerefentry><refentrytitle>btrfs-scrub</refentrytitle>
+          <manvolnum>8</manvolnum></citerefentry>).
+          See
+          <citerefentry><refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>7</manvolnum></citerefentry>
+          for more information on the syntax.
+        '';
+      };
+
+    };
+  };
 
-    boot.initrd.extraUtilsCommands = mkIf inInitrd
+  config = mkMerge [
+    (mkIf enableBtrfs {
+      system.fsPackages = [ pkgs.btrfs-progs ];
+
+      boot.initrd.kernelModules = mkIf inInitrd [ "btrfs" "crc32c" ];
+
+      boot.initrd.extraUtilsCommands = mkIf inInitrd
       ''
         copy_bin_and_libs ${pkgs.btrfs-progs}/bin/btrfs
         ln -sv btrfs $out/bin/btrfsck
         ln -sv btrfsck $out/bin/fsck.btrfs
       '';
 
-    boot.initrd.extraUtilsCommandsTest = mkIf inInitrd
+      boot.initrd.extraUtilsCommandsTest = mkIf inInitrd
       ''
         $out/bin/btrfs --version
       '';
 
-    boot.initrd.postDeviceCommands = mkIf inInitrd
+      boot.initrd.postDeviceCommands = mkIf inInitrd
       ''
         btrfs device scan
       '';
-  };
+    })
+
+    (mkIf enableAutoScrub {
+      assertions = [
+        {
+          assertion = cfgScrub.enable -> (cfgScrub.fileSystems != []);
+          message = ''
+            If 'services.btrfs.autoScrub' is enabled, you need to have at least one
+            btrfs file system mounted via 'fileSystems' or specify a list manually
+            in 'services.btrfs.autoScrub.fileSystems'.
+          '';
+        }
+      ];
+
+      # This will yield duplicated units if the user mounts a filesystem multiple times
+      # or additionally mounts subvolumes, but going the other way around via devices would
+      # yield duplicated units when a filesystem spans multiple devices.
+      # This way around seems like the more sensible default.
+      services.btrfs.autoScrub.fileSystems = mkDefault (mapAttrsToList (name: fs: fs.mountPoint)
+      (filterAttrs (name: fs: fs.fsType == "btrfs") config.fileSystems));
+
+      # TODO: Did not manage to do it via the usual btrfs-scrub@.timer/.service
+      # template units due to problems enabling the parameterized units,
+      # so settled with many units and templating via nix for now.
+      # https://github.com/NixOS/nixpkgs/pull/32496#discussion_r156527544
+      systemd.timers = let
+        scrubTimer = fs: let
+          fs' = utils.escapeSystemdPath fs;
+        in nameValuePair "btrfs-scrub-${fs'}" {
+          description = "regular btrfs scrub timer on ${fs}";
+
+          wantedBy = [ "timers.target" ];
+          timerConfig = {
+            OnCalendar = cfgScrub.interval;
+            AccuracySec = "1d";
+            Persistent = true;
+          };
+        };
+      in listToAttrs (map scrubTimer cfgScrub.fileSystems);
+
+      systemd.services = let
+        scrubService = fs: let
+          fs' = utils.escapeSystemdPath fs;
+        in nameValuePair "btrfs-scrub-${fs'}" {
+          description = "btrfs scrub on ${fs}";
+
+          serviceConfig = {
+            Type = "oneshot";
+            Nice = 19;
+            IOSchedulingClass = "idle";
+            ExecStart = "${pkgs.btrfs-progs}/bin/btrfs scrub start -B ${fs}";
+          };
+        };
+      in listToAttrs (map scrubService cfgScrub.fileSystems);
+    })
+  ];
 }